Application of a simplified calculation for full-wave microtremor H/V spectral ratio based on the diffuse field approximation to identify underground velocity structures

Abstract

Under the diffuse field approximation, the full-wave (FW) microtremor H/V spectral ratio (H/V) is modeled as the square root of the ratio of the sum of imaginary parts of the Green's function of the horizontal components to that of the vertical one. For a given layered medium, the FW H/V can be well approximated with only surface waves (SW) H/V of the "cap-layered" medium which consists of the given layered medium and a new larger velocity half-space (cap layer) at large depth. Because the contribution of surface waves can be simply obtained by the residue theorem, the computation of SW H/V of cap-layered medium is faster than that of FW H/V evaluated by discrete wavenumber method and contour integration method. The simplified computation of SW H/V was then applied to identify the underground velocity structures at six KiK-net strong-motion stations. The inverted underground velocity structures were used to evaluate FW H/Vs which were consistent with the SW H/Vs of corresponding cap-layered media. The previous study on surface waves H/Vs proposed with the distributed surface sources assumption and a fixed Rayleighto-Love waves amplitude ratio for horizontal motions showed a good agreement with the SW H/Vs of our study. The consistency between observed and theoretical spectral ratios, such as the earthquake motions of H/V spectral ratio and spectral ratio of horizontal motions between surface and bottom of borehole, indicated that the underground velocity structures identified from SW H/V of cap-layered medium were well resolved by the new method.

Does the One-Dimensional Assumption Hold for Site Response Analysis? A Study of Seismic Site Responses and Implication for Ground Motion Assessment Using KiK-Net Strong-Motion Data

“…Because downhole logging has been shown to not be fully reliable for some KiK-net sites (Kawase and Matsuo 2004, Wu et al 2017), we need to define quantitative criteria to distinguish sites with properly resolved velocity profiles from others, so that further analyses are not biased by unreliable velocity profiles. For downhole sensors located close to or below the deepest impedance contrast, the reliability of the provided velocity profiles can then be based on the comparison of the fundamental resonance frequencies directly estimated from the empirical amplification function and by indirect modeling of the SH wave transfer function using the given velocity profiles and a correction for the reference rock.…”

Testing the Depths to 1.0 and 2.5 km/s Velocity Isosurfaces in a Velocity Model for Japan and Implications for Ground‐Motion Modeling

“…Thus, site-specific depth measurements can be established from the KiK-net 1D velocity profiles and then used as references to gauge the depth data extracted from the J-SHIS model (Z x−J−SHIS ). But there is one prerequisite for this comparison to be trustworthy, which is that the KiK-net velocity profiles are robust enough to serve as a benchmark as discussed in several recent studies (e.g., Poggi et al, 2012;Wu et al, 2017;Pilz and Cotton, 2019). Therefore, before evaluating the J-SHIS model, we need to identify which KiK-net profiles are appropriate for such testing.…”

Shallow VS Imaging of the Groningen Area from Joint Inversion of Multimode Surface Waves and H/V Spectral Ratios

“…Sánchez-Sesma et al (2011) used the diffuse field assumption (DFA) to link the H/V of the autocorrelated signal (i.e., the directional energy densities in Perton et al, 2009) to the ratio of the imaginary parts of the 1D Green's functions. This formulation naturally allows for inversion of the structure at a site, and the inversion of the full spectrum of the H/V ratio has emerged as a powerful technique that has been applied to diverse data sets and geological settings (Salinas et al, 2014;Spica, Caudron, et al, 2015;Lontsi et al, 2015Lontsi et al, , 2016Rivet et al, 2015;García-Jerez et al, 2016;Piña-Flores et al, 2016;Perton et al, 2017;Wu et al, 2017). It has also proven its potential to give a reliable velocity model of the shallow subsurface using shallow borehole observations from the G-array .…”